Why do some teams have aero helmets?

During the stage 4 TT, why do some teams have those nice aero helmets and others have the standard large vented ones? Is that cause some teams can't afford it or cause the helmet companies won't sell it to them?

This year the aero helmets have to be a safety helmet and not just a piece of plastic as they have used in other years, so this years helmets are still in the design phase. Laser's helmet used by quick-step and lotto-domo is rather large, bulky, and goofy looking (it looks like a big plastic mullet) and some riders opted not to wear them in the TTT.